1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or copier which forms a color image by an electrophotographic process and an image forming method, and more particularly, to an image forming apparatus which performs an intermediate transfer process to overlay-transfer respective color toner images, formed on plural photoconductor drums, onto an intermediate transfer belt and then finally transfer the images onto a print sheet.
2. Description of Related Art
Conventionally, image forming apparatuses such as a printer which form a color image by using an electrophotographic process are roughly classified into 4-pass type and single-pass type (tandem type) apparatuses.
FIG. 1 shows a conventional 4-pass type process. The 4-pass type image forming apparatus has a single photoconductor drum 100 and a developing unit 106 for forming yellow (Y), magenta (M), cyan (C) and black (K) color images. The surface of the photoconductor drum 100 is uniformly charged by a charger 102 in the rear of a cleaning blade 101, and an electrostatic latent image is formed by laser scanning by an exposure unit 104. Next, a yellow toner image is formed by development using yellow toner in a developing unit 106, and the toner image is electrostatically transferred onto a transfer belt 108 as an intermediate transfer medium in contact with the photoconductor drum 100 by application of primary transfer voltage VT1 by a transfer roller 110. Then, the same processing is repeated for magenta, cyan and black colors and the respective color toner images are overlaid on the transfer belt 108. Finally, the 4 color developers are transferred onto a print sheet at a time by a transfer roller 111 to which a secondary transfer voltage VT2 is applied, and the image is fixed onto the print sheet by a fixer 112.
Since electric charge is accumulated on the transfer belt 108 and the print sheet, the potential on the transfer belt 108 after transfer shows a mild attenuation characteristic. In the case of the 4-pass type process, the next transfer is performed after one rotation of the transfer belt. As shown in FIG. 2, there is sufficient time between transfer at time t1 and the next transfer at time t2. Since a toner potential 114 and a transfer belt potential 116 by a transfer voltage VT1 are sufficiently attenuated during this time interval, the application of the same transfer voltage VT1 can be repeated 4 times.
In this manner, the case of the 4-pass type image forming apparatus, which merely has the photoconductor drum 100, the cleaning blade 101, the charger 102, the exposure unit 104 and the transfer roller 110, is advantageous in terms of cost. However, to form one color image, the intermediate transfer belt 108 must be rotated 4 times, and the speed of color printing is ¼ of that of monochrome printing.
FIG. 3 shows a conventional single-pass type (tandem type) process (Japanese Published Unexamined Patent Application No. Hei 11-249452). In the single-pass type image forming apparatus, image forming units 118-1 to 118-4 are arrayed for respective yellow (Y), magenta (M), cyan (C) and black (K) colors. That is, the image forming units 118-1 to 118-4 have photoconductor drums 120-1 to 120-4 and cleaning blades, chargers, LED exposure units and developing units around the drums, and the image forming units 118-1 to 118-4 form respective color images. The respective color images formed on the photoconductor drums 120-1 to 120-4 are electrostatically and sequentially overlay-transferred onto an intermediate transfer belt 116 which turns while it is in contact with the respective color photoconductor drums 120-1 to 120-4 by application of transfer voltage by transfer rollers 122-1 to 122-4. Finally, the overlaid color images are transferred onto a print sheet at a time by application of transfer voltage by a paper transfer roller 134 provided on the opposite side of a backup roller 132, and fixed to the print sheet by a fixer 122, thus a color image is obtained.
As the transfer belt 116 is used as an intermediate transfer medium, the transfer from the photoconductor drum to the intermediate transfer belt is generally referred to as primary transfer, and the transfer from the intermediate transfer belt to the print sheet, secondary transfer. Further, generally, the transfer rollers 122-1 to 122-4 for the transfer from the photoconductor drums 120-1 to 120-4 to the intermediate transfer belt 116 and the paper transfer roller 134 for the transfer from the intermediate transfer belt 116 to the print sheet are conductive sponge rollers.
In the case of the single-pass type process in the above arrangement, a color image can be formed by one pass, the print speed is faster than that in the case of the 4-pass type process.
FIG. 4 shows a potential attenuation curve of the intermediate transfer belt in the single-pass type process in FIG. 3. In the single-pass type apparatus, yellow, magenta, cyan and black color toner images are developed on the respective photoconductor drums 120-1 to 120-4 and sequentially transferred onto the intermediate transfer belt 116. First, at time t1, a transfer voltage VT is applied as a yellow transfer voltage VTY and the yellow image is transferred from the photoconductor drum 120-1 to the intermediate transfer belt 116, then a potential 144-1 on the belt shows a mild attenuation characteristic since electric charge is accumulated on the intermediate transfer belt 116. A residual potential ΔV2 remains upon the next transfer from the magenta photoconductor drum 120-2. Accordingly, to obtain an effective transfer voltage VT for the magenta image on the photoconductor drum 120-2 at time t2, a transfer voltage VTM must be increased by the residual potential ΔV2. Similarly, a cyan transfer voltage VTC at time t3 and a black transfer voltage VKT at time t4 must be increased by respective residual potentials ΔV3 and ΔV4. For this reason, in the single-pass type image formation process using the intermediate transfer belt, the transfer voltage must be set to appropriate values for the respective colors. As a result, 4 specialized high-voltage power sources must be provided for the 4 colors, and further, 1 high-voltage power source must be provided for the secondary transfer, i.e., total 5 high-voltage power sources must be provided. Thus the transfer power sources are complicated and the costs are increased.
On the other hand, in both types of image forming processes, in color image formation by overlay-transferring colors onto a print sheet or an intermediate transfer medium, upon transfer from secondary colors except monochrome primary color, as toner is overlaid on a previous color toner, a higher transfer voltage than that for the primary color is required. Since the previous color toner has an electric charge, the transfer electric field is weakened upon transfer of the next toner. Generally, a voltage margin (voltage allowance) of transfer efficiency is designed to have allowance to a certain degree. If the voltage margins of transfer efficiencies for the primary to tertiary colors overlap with each other, transfer from the primary to tertiary colors can be excellently performed.
However, it is difficult to ensure a voltage margin to satisfy the transfer from the primary to tertiary colors and to increase the reliability of transfer characteristics. For this purpose, the following various methods have been proposed or performed.
(1) Reduction of Toner Adhesion Amount
In color-overlay transfer, it is the most difficult to perform transfer to generate black color as a tertiary color by overlaying yellow, magenta and cyan. Accordingly, so-called under color removal (UCR) is often performed to replace color toner with black toner at 100% or some percentage. In this case, the color reproduction range of a color image formed by use of 3 colors is narrowed.
(2) Optimization of Each Color Toner Charging Amount
Optimization of each color toner charging amount is known (Japanese Published Unexamined Patent Application Nos. Hei 6-202429, Hei 8-106197 and Hei 10-207164). However, in this method, as toner charging amounts are different, it is necessary to optimize developing conditions for respective colors, and further, it is necessary to determine toner manufacturing methods for respective colors.
(3) Control of Toner Charging Amount Before Transfer
Charging toner by a non-contact charger to obtain an optimum charging amount for overlay-transfer prior to the overlay transfer is known (Japanese Published Unexamined Patent Application No. Hei 8-15947). In this method, as another charger is required, the costs for the charger and power source used for the charger are increased, and further, as the space for the charger must be ensured, the apparatus is upsized.
(4) Optimization of Transfer Voltage
Optimization of transfer voltage for each color to attain stable transfer is known (Japanese Published Unexamined Patent Application No. Hei 11-202651). In this method, in the case of tandem type process, the power source is required for each color, and the costs are increased.